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The Hubble Space Telescope has captured images of three odd galaxies that may help scientists solve a 13 billion-year cosmic mystery.

The galaxies are so old and faint that astronomers nicknamed them "ghost galaxies" in a description. The objects are among the smallest and faintest galaxies near our own Milky Way galaxy, researchers said.

"These galaxies are fossils of the early universe: they have barely changed for 13 billion years," scientists explained in a July 10 announcement. "The discovery could help explain the so-called 'missing satellite' problem, where only a handful of satellite galaxies have been found around the Milky Way, against the thousands that are predicted by theories."

The three galaxies observed by the Hubble telescope are known as Hercules, Leo IV and Ursa Major. All three objects are small dwarf galaxies that appear to have begun forming about 13 billion years ago and then ? for an unknown reason ? their growth hit a cosmic wall. Since the universe is estimated to be about 13.7 billion years old, the galaxies were born sometime within the first billion years of the cosmos.

"These galaxies are all ancient and they?re all the same age, so you know something came down like a guillotine and turned off the star formation at the same time in these galaxies," said study leader Tom Brown of the Space Telescope Science Institute in Baltimore, Md. "The most likely explanation is a process called reionization."

In the history of the universe, the reionization period marks a time when the cosmos transformed from being filled with cool neutral hydrogen (which carried no charge) into a universe with ionized hydrogen that had been split into its component electrons and protons. That change made the hydrogen fog of the early universe transparent to ultraviolet light.

The universe was filled with the neutral hydrogen about 300,000 years after the Big Bang, with the reionization period occurring sometime in the 1 billion years that followed, astronomers have said. Scientists suspect that radiation from the first stars and galaxies caused the reionization.

In the new study, Brown and his colleagues found that the same radiation that triggered the reionization of the universe may have also stunted star formation in dwarf galaxies such as those spotted in the new Hubble telescope views.